Magnetic recording medium

ABSTRACT

A magnetic recording medium having a magnetic layer on at least one side of a base film which is a stretched film of styrene polymer having a high degree of syndiotactic configuration or a composition thereof or a laminate film containing the stretched film, wherein linear expansion coefficient is not more than 5×10 -5  /° C. and static friction coefficient is not less than 0.3 and not more than 1.0 is disclosed. 
     The magnetic recording medium has good sliding properties, smoothness, heat resistance and moisture resistance, is excellent in dimensional stability to temperature and moisture, and has good coersive force, thus can be highly densified.

FIELD OF THE INVENTION

The present invention relates to magnetic recording medium, magnetictape, magnetic disk and magnetic card, more specifically, to magneticrecording medium which is excellent in physical properties such assliding properties, smoothness, heat resistance, moisture resistance,dimensional stability, electrical insulating properties and furthercapable of densification, and magnetic tape and magnetic disk, andmagnetic card which have these properties and high whiteness degree.

BACKGROUND OF THE INVENTION

Hitherto, polyethylene terephthalate (PET) film substrate coated withmagnetic layer has been widely used as a magnetic recording medium.However, PET film is inferior in water resistance, and sometimes it cannot be used because it may be hydrolyzed at high temperature under highhumidity. Glass transition temperature of PET is 60° to 80° C. Elasticmodulus may be sharply decreased at a temperature higher than the aboverange, and elongation or the like may readily occur. Accordingly,reliability is not always sufficient. Magnetic tape using such PET filmas a base film has problem, that is, bad reproduction due to elongationof the base film at high temperature, for example, in a car.

As a base film for magnetic disk such as floppy disk demand for whichhas been increasing, the above PET film has been used. Recently, asminiaturization and densification of floppy disk, further improvement ofphysical properties such as heat resistance, moisture resistance,dimensional stability and the like has been required. However, amagnetic disk using the above PET film as a base material has problemsin dimensional change with change of moisture, anisotropism of thermalexpansion coefficient in face, eternal shrinkage when left at hightemperature under high moisture over a long period of time.Polyphenylene sulfide (PPS) film used as a base film for this magneticdisk may overcome the above problems, but it is too expensive forpractical use. Satisfactory base materials for magnetic disk have notbeen proposed, yet.

On the other hand, magnetic recording medium has been further densified,and demand for its reliability has been increasing. One of the means fordensification is a method wherein a magnetic layer is laminated on abase film by a vapor deposition method. This method, however, hasproblems, for example, insufficient heat resistance, separation ofoligomer when PET film is used as a base film.

Further, demand for magnetic card has been increasing. As base films formagnetic cards, polyvinyl chloride film and polyester film have beenknown. However, base film of polyvinyl chloride has low heat resistanceand less resilient. Base film of polyester has problems in practicalapplication, for example, insufficient whiteness, too much gloss. Basefilms for magnetic cards which are excellent in heat resistance,chemical resistance, electrical insulating properties, dimensionalstability and the like and have sufficient whiteness are required.

As described above, the magnetic recording medium obtained from theconventional base film have various problems for practical application,for example, insufficient sliding properties, smoothness, heatresistance, moisture resistance, dimensional stability and the like.

DISCLOSURE OF THE INVENTION

The present inventors have studied to develop a magnetic recordingmedium which overcomes the above problems. Previously, the group of thepresent inventors have succeeded to develop a styrene polymer having ahigh degree of syndiotactic configuration. This polymer, which issuperior to PET in heat resistance, water resistance, dimensionalstability and the like, is a polymer with less oligomer and expected tobe widely applied. The present inventors have studied intensively todevelop a highly-densified magnetic recording medium with highreliability even under severe conditions such as high temperature, highhumidity using this styrene polymer having a high degree of syndiotacticconfiguration.

The object of the present invention is to provide a magnetic recordingmedium excellent in physical properties such as heat resistance,moisture resistance, smoothness, dimensional stability, and high densitymagnetic recording medium which has the above properties and is furtherexcellent in, for example, coercive force.

Another object of the present invention is to provide heat resistantmagnetic tape excellent in heat resistance, moisture resistance,smoothness, dimensional stability, mechanical strength, and a magneticdisk which is especially excellent in heat resistance, moistureresistance, smoothness, dimensional stability and the like.

Still another object of the present invention is to provide a magneticcard which is excellent in, for example, heat resistance, moistureresistance, smoothness, mechanical strength, and has high whiteness.

As a result of study from such point of view, it has been found thatstretched film of the above styrene polymer or a composition thereof, orlaminated film containing said stretched film, which has a magneticlayer in a base film, the physical properties of the film, for example,linear expansion coefficient, static friction coefficient, surfaceroughness, elastic modulus, heat deformation temperature being in acertain range, may provide magnetic recording medium, magnetic tape,magnetic disk which meet the above object.

Further, when a magnetic card which is required for printing is preparedfrom a stretched film comprising styrene polymer having a high degree ofsyndiotactic configuration, the product is excellent in, for example,heat resistance, chemical resistance, electrical insulating properties,but is disadvantageous because gloss is too high and whiteness isinsufficient. As a result of a further study, it has been found that amagnetic card with sufficient whiteness can be obtained withoutdeterioration of the intrinsic properties by compounding a whiteinorganic filler in a specific range in the styrene polymer having ahigh degree of syndiotactic configuration.

The present invention has been attained based on such findings.

That is, the present invention provides a magnetic recording mediumwhich has a magnetic layer on at least one side of a base film which isa stretched film of a styrene polymer having a high degree ofsyndiotactic configuration or a composition thereof, or a laminated filmcontaining said stretched film, and has linear expansion coefficient ofnot more than 5×10⁻⁵ /° C. and static friction coefficient of 0.3 to1.0. The present invention provides a heat resistant magnetic tapecomprising a base film which is a stretched film of styrene polymerhaving a high degree of syndiotactic configuration or a compositionthereof or a laminated film containing said stretched film, whereinelastic moduli at room temperature in machine direction (MD) and intransverse direction (TD) are at least 30,000 kg/cm², the ratio (MD/TD)is at least 0.8, elastic modulus at 90° C. in MD is at least 10,000kg/cm², linear expansion coefficient at 0° to 90° C. is not more than5×10⁻⁵ /° C., and surface roughness Ra, is 0.001 to 0.3, and a magneticlayer provided at least on one side thereof. Further, the presentinvention provides a magnetic disk comprising a base film which is astretched film of a styrene polymer having a high degree of syndiotacticconfiguration or a composition thereof or a laminated film containingsaid stretched film, wherein linear expansion coefficient in MD (α_(MD))and in TD (α_(TD)) is not more than 5×10⁻⁵ /° C., the ratio (α_(MD)/α_(TD)) is 0.3 to 3, surface roughness Ra is 0.001 to 0.03 μm, andthickness is 20 to 200 μm, and a magnetic layer provided at least oneside thereof. Moreover, the present invention provides a magnetic cardhaving a magnetic layer on at least one side of a base film which is afilm comprising 40 to 99% by weight of a film of a styrene polymerhaving a high degree of syndiotactic configuration or a compositionthereof or a laminated film containing said film wherein linearexpansion coefficient is not more than 7×10⁻⁵ /° C., whiteness is notless than 75, thickness is 50 to 1,000 μm.

BEST MODE TO CONDUCT THE INVENTION

In the present invention, as a base film of a magnetic recording medium,a stretched film which comprises a monolayer film comprising the abovestretched film of the styrene polymer having a high degree ofsyndiotactic configuration or a laminate film containing said stretchedfilm is used. This base film has linear expansion coefficient of notmore than 5×10⁻⁵ /° C., preferably not more than 4×10⁻⁵ /° C., andstatic friction coefficient, μs, of 0.3 to 1.0, preferably 0.3 to 0.9.This static friction coefficient, μs, determines the surface conditionof the base film. Especially, static friction coefficient μs of theopposite side to the side, on which magnetic layer is formed, of thebase film is desirably in the above range.

Surface roughness Ra of the surface of this base film is notparticularly limited, but surface roughness Ra of at least one sidethereof is preferably 0.001 to 0.05 μm, most preferably, 0.001 to 0.04μm. For production of high densified magnetic recording medium, surfaceroughness Ra is preferably 0.001 to 0.03, particularly, 0.005 to 0.02μm. Especially, surface roughness Ra of the base film on the magneticlayer-formed side is desirably in the above range.

When a magnetic layer is, for example, vapor deposited for densificationof the aforementioned base film, the film having heat deformationtemperature of not less than 230° C., particularly not less than 235° C.is the most preferable.

Moisture expansion coefficient of the above base film is notparticularly limited, but preferably not more than 5×10⁻⁵ /% RH.

As the base film for the magnetic tape of the present invention, amonolayer film consisting of the aforementioned stretched film or alaminate film containing said stretched film is used. The physicalproperties of this base film may be as follows: elastic modulus at roomtemperature (about 0° C. to 40° C.) in MD and TD is not less than 30,000kg/cm², preferably not less than 33,000 kg/cm², the ratio of elasticmodulus in MD to elastic modulus in TD (MD/TD) is not less than 0.8,preferably, 0.8 to 6, and elastic modulus in MD at 90° C. is not lessthan 10,000 kg/cm², preferably, not less than 15,000 kg/cm², linearexpansion coefficient at 0° to 90° C. is not more than 5×10⁻⁵ /° C.,preferably, not more than 4×10⁻⁵ /° C., and surface roughness Ra is0.001 to 0.03, preferably, 0.003 to 0.03.

When elastic modulus in MD at room temperature is less than 30,000kg/cm², the tape may be stretched with a little load on running and goodreproduction may not be obtained. When elastic modulus in TD at roomtemperature is less than 30,000 kg/cm², the tape may readily folded,sometimes resulting in bad reproduction. When the ratio of elasticmodulus in MD to that in TD, i.e., MD/TD is less than 0.8, the tape maybe readily torn off.

When elastic modulus in MD at 90° C. is less than 10,000 kg/cm², theresulting tape can not be used at the high temperature, i.e. about 90°C. When linear expansion coefficient is more than 5×10⁻⁵ /° C., theresulting tape can not be used at the high temperature, i.e. about 90°C. When surface roughness Ra is outside of the range of 0.001 to 0.3,good running conditions as magnetic tape may not be obtained.

A monolayer film comprising the aforementioned stretched film or alaminate film containing said stretched film may be used as a base filmof the magnetic disk of the present invention. The physical propertiesof this base film are as follows: Linear expansion coefficient in MD(α_(MD)) and that in TD (α_(TD)) are not more than 5×10⁻⁵ /° C.,preferably, not more than 4×10⁻⁵ /° C., and the ratio thereof (α_(MD)/α_(TD)) is 0.3 to 3, preferably, 0.5 to 2, and surface roughness is0.001 to 0.03 μm, preferably, 0.005 to 0.03 μm, and thickness is 20 to200 μm, preferably, 50 to 150 μm.

The film with α_(MD) or α_(TD) over 5×10⁻⁵ /° C. is not practicalbecause it is deficient in dimensional stability. When the film withα_(MD) /α_(TD) below 0 3 or over 3 is used as a magnetic disk, the diskmay have isotropic dimensional stability in the surface uponenvironmental change, and good reproduction can not be obtained. Surfaceroughness Ra over 0.03 μm produces problems such as abrasion of themagnetic head. Ra below 0.001 μm produces problems in slidingproperties, and tape with good running condition can not be readilyobtained. Too thin base film provides magnetic disk deficient inresilience and self-supporting property which may be necessary for amagnetic disk, and the product can not be practically used, on the otherhand too thick film is deficient in flexibility.

For production of a base film of such magnetic recording medium,magnetic tape and magnetic disk, various methods can be employed. Forexample, three methods shown below can be employed.

(1) A method wherein a composition comprising a styrene polymer having ahigh degree of syndiotactic configuration compounded with an inorganicfiller, particularly, 0.001 to 1% by weight of an inorganic filler withan average particle diameter of 0.01 to 3 μm, is heat-melted, extruded,cooled and solidified, heated, stretched and heat-treated to form astretched film, thereby providing a readily slidable monolayer filmcomprising this stretched film alone and whose both sides are smooth androughened.

(2) A method wherein the composition used in the above (1) and a styrenepolymer having a high degree of syndiotactic configuration,particularly, a styrene polymer of high purity with residual aluminumcontent of not more than 3,000 ppm, residual titanium content of notmore than 10 ppm and residual styrene monomer content of not more than7,000 ppm or a composition containing this styrene polymer as a mainingredient (containing no inorganic filler) are heat melted,co-extruded, cooled and solidified, heated, stretched to form a laminateof two or more stretched films. Accordingly, a readily slidablemultilayer (laminate) film whose one side is extraordinary smooth andthe other face is toughened may be obtained.

(3) A method wherein a styrene polymer having a high degree ofsyndiotactic configuration, particularly, a styrene polymer of highpurity with residual aluminum content of not more than 3,000 ppm,residual titanium content of not more than 10 ppm and residual styrenemonomer content of not more than 7,000 ppm, or a composition containingthis styrene polymer as a main ingredient (containing no inorganicfiller) is heat-melted, extruded, cooled and solidified, heated,stretched and heat-treated, while a layer comprising resin compositioncontaining resin which can roughen the surface or an extremely fineinorganic filler is formed onto at least one face thereof by lamination,coating, vapor deposition or the like to give a readily slidablemultilayer film wherein one face is extremely smooth and the other faceis roughened. In this case, the layer comprising the above resincomposition which may be stretched as needed can be used.

As the resin used for lamination, a styrene polymer having a high degreeof syndiotactic configuration or various blend resins may be used. Amongthem, resins with high melting point or softening point are preferred.As for type, amount and particle diameter of the inorganic fillercontained in the other resins, those described below is preferably used.

The method for lamination is not particularly limited, but, in additionto the co-extrusion method, a process for production of stretched filminvolving a method for dissolving and coating the other resin and resincomposition thereof provides good productivity.

Particularly in tape, the above layer having a rough surface is used asa back coat layer, and a magnetic layer is mainly formed on the oppositeside, i.e., extremely smooth surface.

Thus obtained stretched film is 2 to 500 μm thick with linear expansioncoefficient of not more than 5×10⁻⁵ /° C. and static frictioncoefficient μs of 0.3 to 1.0. The surface roughness Ra of the both sidesaccording to the method (1) is 0.005 to 0.05 μm, and according to themethods (2) and (3) the layer have both readily slidable rough surfaceand smooth surface, and surface roughness of said surfaces is 0.005 to0.05 μm and 0.001 to 0.04 μm, respectively. Selecting conditions forstretching and additives and the like, a film with surface roughness of0.001 to 0.03 μm, elastic modulus at room temperature in MD and TD of atleast 30,000 kg/cm², the ratio (MD/TD) of at least 0.8, elastic modulusat 90° C. in MD of at least 10,000 kg/cm², linear expansion coefficientin MD (α_(MD) ) and in TD (α_(TD)) of not more than 5×10⁻⁵ /° C., andthe ratio α_(MD) /α_(TD) of 0.3 to 3 may be readily obtained. Generally,heat deformation temperature of the resulting stretched film is at least230° C., and more preferably, moisture expansion coefficient is not morethan 5×10⁻⁵ /% RH.

As for the thickness of the film, films of 2 to 20 μm thick, 20 to 150μm thick and 100 to 500 μm thick are suitably used as a magnetic tape, amagnetic disk and a magnetic card, respectively.

Magnetic recording medium may be prepared using these films.Particularly in the production of magnetic disk, film whose surfaceadhesion is improved by corona treatment or the like may be used asneeded. Further, an adhesive layer may be provided on the side on whicha magnetic layer is to be provided.

Here, a styrene polymer having a high degree of syndiotacticconfiguration used as a raw material for the above film means a styrenepolymer wherein stereochemical structure is a high degree ofsyndiotactic configuration, that is, the stereostructure in which phenylgroups or substituted phenyl groups as side chains are locatedalternately in opposite directions relative to the main chain consistingof carbon-carbon bonds. Tacticity is quantitatively determined by thenuclear magnetic resonance method (¹³ C-NMR method) using carbonisotope. The tacticity measured by the ¹³ C-NMR method can be indicatedin terms of proportions of structural units continuously connected toeach other, i.e., a diad in which two structural units are connected toeach other, a triad in which three structural units are connected toeach other and a pentad in which five structural units are connected toeach other. The styrene polymer having a high degree of syndiotacticconfiguration in the present invention means styrene polymer,poly(alkylstyrene), poly(halogenated styrene), poly(alkoxystyrene),poly(vinyl benzoate), hydrogenated polymers thereof and a mixturethereof, or copolymers containing these structural units each havingsuch syndiotacticity that the proportion of racemic diad is at least75%, preferably at least 85%, or proportions of racemic pentad is atleast 30% and preferably at least 50%. The poly(alkylstyrene) includespoly(methylstyrene), poly(ethylstyrene), poly(propylstyrene),poly(butylstyrene), poly(phenylstyrene), poly(vinylnaphthalene),poly(vinylstyrene), poly(acenaphthylene); and the poly(halogenatedstyrene) includes poly(chlorostyrene), poly(bromostyrene) andpoly(fluorostyrene). The poly(alkoxystyrene) includespoly(methoxystyrene), poly(ethoxystyrene). Of these, a particularlypreferred styrene polymer includes polystyrene, poly(p-methylstyrene),poly(m-methylstyrene), poly(p-tert-butylstyrene), poly(p-chlorostyrene),poly(m-chlorostyrene), poly(p-fluorostyrene) and further a copolymer ofstyrene and p-methylstyrene (see Japanese Patent Application Laid-OpenNo. 187708/1987).

Comonomer of the styrene copolymer includes, in addition to theabove-described monomer of styrene polymer, olefin monomer such asethylene, propylene, butene, hexene, octene; diene monomer such asbutadiene, isoprene; cyclic diene monomer or polar vinyl monomer such asmethyl methacrylate, maleic anhydride, acrylonitrile.

Molecular weight of the styrene polymer is not particularly limited, butthe styrene polymers having weight average molecular weight of 10,000 to3,000,000, especially, 50,000 to 1,500,000 are most suitable. Whenweight average molecular weight is less than 10,000, the product may notbe sufficiently stretched. Further, the range of molecular-weightdistribution is not limited and various ranges can be used. The value,weight average molecular weight (Mw)/number average molecular weight(Mn) is preferably 1.5 to 8. The styrene polymer having a high degree ofsyndiotactic configuration is much superior in heat resistance to theconventional styrene polymer having an atactic configuration.

The styrene polymer having a high degree of syndiotactic configurationwhich constitutes the aforementioned stretched film (especially, a filmhaving an extremely smooth surface) is as described above. Especially,the styrene polymer, wherein the residual aluminum content in thestyrene polymer is not more than 3,000 ppm, the residual titaniumcontent is not more than 10 ppm and the residual styrene monomer contentis not more than 7,000 ppm, is preferred. Particularly, the styrenepolymer wherein the residual aluminum content is not more than 1,000ppm, the residual titanium content is not more than 7 ppm and theresidual styrene monomer content is not more than 5,000 ppm is mostpreferable.

The methods for production of such styrene polymer of high purityinclude various ones as shown below. Firstly, for control of theresidual aluminum content and the residual titanium content within theabove range, (1) a method in which a highly active catalyst is used toproduce a styrene polymer (see, Japanese Patent Application Laid-OpenNo. 294705/1989) or (2) a method which comprises deashing and washing,that is, a method wherein a styrene monomer is polymerized using aconventional organometallic compound of group IVA described in JapanesePatent Application laid-Open No. 187708/1987 and the like andalkylaluminoxane such as methylaluminoxane as the catalyst components,then the resulting styrene polymer having a syndiotactic configurationis deashed with a solution of acid or alkali in a suitable solvent, andwashed with a suitable solvent.

As mentioned above, a styrene polymer having a syndiotacticconfiguration with less residual aluminum and residual titanium contentcan be obtained by the method (1) or (2). Further, the product istreated by the following method (3) or (4) to control the residualstyrene monomer content below 7,000 ppm.

(3) A method wherein the above styrene polymer is dried under reducedpressure.

For drying under reduced pressure, it is efficient to set the dryingtemperature at the glass transition temperature of the polymer orhigher.

(4) A method wherein the above styrene polymer is degassed by anextruder.

The above styrene polymer or the styrene polymer dried under reducedpressure by the method (3) is degassed by an extruder and simultaneouslyformed into a material for molding (pellet). An extruder used in thisstep is preferably equipped with a vent. Either a uniaxial or biaxialextruder may be used.

Such treatment provides a styrene polymer of high purity which containsless residual aluminum, residual titanium and residual styrene monomerand has a syndiotactic configuration.

The component of the stretched film having a readily slidable roughsurface is a composition wherein a styrene polymer having a high degreeof syndiotactic configuration is compounded with an inorganic filler,particularly the above styrene polymer containing 0.001 to 1% by weightof inorganic filler with an average particle diameter of 0.01 to 3 μmThis composition may be prepared by compounding the styrene polymer(which is not necessarily of high purity as described above but obtainedaccording to the method described in Japanese Patent ApplicationLaid-Open No. 187708/1987) with an inorganic filler or as a deposit inthe polymer. In this case, the inorganic filler means oxide, hydroxide,sulfide, nitride, halide, carbonate, acetate, phosphate, phosphite,organic carboxylate, silicate, titanate or borate of the group IA, IIA,IVA, VIA, VIIAT VIII, IB, IIB, IIIB or IVB element, and hydrate compoundthereof, complex compound and natural mineral particles containing themas major ingredient.

For example, group IA element compound such as lithium fluoride, borax(hydrate salt of sodium borate); group IIA element compound such asmagnesium carbonate, magnesium phosphate, magnesium oxide (magnesia),magnesium chloride, magnesium acetate, magnesium fluoride, magnesiumtitanate, magnesium silicate, hydrate salt of magnesium silicate (talc),calcium carbonate, calcium phosphate, calcium phosphite, calcium sulfate(gypsum), calcium acetate, calcium terephthalate, calcium hydroxide,calcium silicate, calcium fluoride, calcium titanate, strontiumtitanate, barium carbonate, barium phosphate, barium sulfate, bariumphosphite; group IVA element compound such as titanium dioxide(titania), titanium monooxide, titanium nitride, zirconium dioxide(zirconia), zirconium monooxide; group VIA element compound such asmolybdenum dioxide, molybdenum trioxide, molybdenum sulfide; group VIIAelement compound such as manganese chloride, manganese acetate; groupVIII element compound such as cobalt chloride, cobalt acetate: group IBelement compound such as copper iodide; group IIB element compound suchas zinc oxide, zinc acetate; group IIIB element compound such asaluminum oxide (alumina), aluminum hydroxide, aluminum fluoride,aluminosilicate (alumina silicate, kaolin, kaolinite); group IVB elementcompound such as silicon oxide (silica, silica gel), plumbago, carbon,graphite, glass; particulate natural mineral such as carnallite,kainite, isinglass (mica, phlogopite) and pyrolusite.

The average particle diameter of the inorganic filler to be used in thepresent invention is not particularly limited, but preferably, 0.01 to 3μm, more preferably, 0.01 to 1 μm. The content in the molded product is0.001 to 1% by weight, preferably, 0.001 to 0.6% by weight. When theaverage particle diameter is less than 0.01 μm, it may be difficult todisperse particles because of secondary agglomeration of particles. Whenthe average particle diameter is more than 3 μm, smoothness may bedeteriorated. If the content of the inorganic filler in the compositionis less than 0.001% by weight, the effect of improvement of slidingproperty may be insufficient, and if the content is more than 1% byweight, stretching of thin film may become difficult.

The aforementioned inorganic filler is a component which is effective toattain the object of the present invention. But it may contain othertype fine particle or a fine particle with different particle diameter,or inorganic filler or the like so long as the object of the presentinvention is not hindered.

The inorganic filler is contained in the final molded product (film) andthe method for compounding is not limited. For example, it is added orobtained as a deposit in the optional step during polymerization, oradded in the optional step during melt extrusion.

Particularly, in the present invention, a method wherein the aboveinorganic filler as slurry is added in the optional step duringpolymerization process is preferable to inhibit secondary agglomerationof particles.

For effective dispersion of these fillers, dispersant, surfactant or thelike may be used.

Considering moldability, mechanical properties, surface properties andthe like, antioxidant, antistatic agent, flame retardant, inorganicfiller, or other resin may be optionally compounded with the rawmaterial used in the stretched film which constitutes the base film ofthe present invention, so long as the object of the present invention isnot hindered.

The other resin includes various kinds, for example, styrene polymerhaving an atactic configuration, styrene polymer having an isotacticconfiguration, polyphenylene ether and the like. Such resins may bereadily compatibilized with the above styrene polymer having asyndiotactic configuration and effective to control crystallization whenpre-molded product for stretching is prepared, thereby providing a filmwith enhanced stretching properties and excellent mechanical properties,whose stretching conditions may be readily controlled. Among them, whenstyrene polymer having an atactic and/or isotactic configuration iscompounded, it is preferably composed of the same monomers as those ofthe styrene polymer having a syndiotactic configuration. The content ofthe compatible resin component is 70 to 1% by weight, preferably, 50 to2% by weight. When the content of the compatible resin component exceeds70% by weight, heat resistance, which is an advantage of the styrenepolymer having a syndiotactic configuration, may be undesirably spoiled.The other noncompatible resins which can be added to the polymer of thepresent invention include a polyolefin such as polyethylene,polypropylene, polybutene, polypentene; a polyester such as polyethyleneterephthalate, polybutylene terephthalate, polyethylene naphthalate; apolyamide such as nylon-6, nylon-6,6; a polythioether such aspolyphenylene sulfide, a polycarbonate, a polyarylate, a polysulfone, apolyether ether ketone, a polyethersulfone, a polyimide, a halogenatedvinyl polymer such as Teflon, an acrylic polymer such as polymethylmethacrylate, a polyvinyl alcohol, and all but the aforementionedcompatible resins. There are also cross linked resins containing theaforementioned compatible resins. When the styrene polymer of thepresent invention having a syndiotactic configuration contains a smallamount of the resin, such resin, which is incompatible with the abovestyrene polymer, can be dispersed like islands in the styrene polymerhaving a syndiotactic configuration. Accordingly, it is effective toprovide proper gloss and to improve sliding property of the surfaceafter stretching. The content of these non-compatible resin componentsis 50 to 2% by weight for the purpose of providing gloss, and 0.001 to5% by weight for the purpose of controlling the surface properties. Whenthe temperature at which the product is used is high, non-compatibleresin with considerable heat resistance is preferably used.

Base film of the magnetic recording medium, magnetic tape, magnetic diskof the present invention can be formed according to the aforementionedmethods. The operations from heat melting to heat treatment (annealing)will be explained in detail.

Firstly, the above material for molding is usually extruded (orcoextruded) to give a pre-molded product for stretching (film, sheet ortube). In this molding, the aforementioned heat-melted material formolding is generally molding into a desired form by an extruder.Alternatively, the material for molding may be molded without heatmelting while it is softened. An extruder used in this case may beeither a uniaxial extruder or a biaxial extruder, with or without vent.A uniaxial tandem type is preferred. By using an extruder with asuitable mesh, impurities and contaminants can be removed. Preferably,for production of a stretched film having a smooth surface, the mesh ispreferably at least 100 mesh, most preferably, at least 400 mesh.Considering pressure resistance and strength of the mesh itself, meshwith gauge lower than the above may be placed before or behind thereof.As for the shape of the mesh, for example, plane or cylinder mesh may beproperly selected and used.

The extrusion conditions are not particularly limited and properlyselected depending on the various circumstances. Preferably, thetemperature is selected in the range from melting point to thetemperature 50° C. higher than decomposition temperature of the materialfor molding, and shear stress is not more than 5×10⁶ dyne/cm². The dieused is a T-die, a ring die or the like.

After the above extrusion, the resulting pre-molded product is cooledand solidified. As a refrigerant used in this step, for example, gas,liquid, metal roller and the like may be used. When a metal roller isused, a method using air knife, air chamber, touch roll, electrostaticapplication and the like Is effective to prevent uneven thickness andsurge.

The temperature of cool solidification is generally 0° C. to 30° C.higher than glass transition temperature of the premolded product forstretching, preferably from 20° C. to glass transition temperature. Thecooling rate is properly selected within the range from 200° to 3 °C./sec.

In the present invention, the cooled and solidified pre-molded productis preferably uni- or bi-axially stretched. For biaxial stretching,transverse direction (TD) stretching and machine direction (MD)stretching may be simultaneously conducted, or successively conducted insuitable order. Alternatively, stretching may be conducted in one step,or in multiple steps.

Methods for stretching include various methods such as a method using atenter, a method wherein the product is stretched between rollers, amethod by bubbling using a pressure of a gas, a method by rolling andthe like. These methods may be applied singly or in combination.Especially, a method comprising MD stretching between rollers andsubsequent TD stretching by a tenter, optionally followed byre-stretching, is preferable to reduce surface roughness, Ra. In thesuccessive stretching, the absolute value of birefringence |Δn| of MDstretching during the primary step should be 3×10³ to 70×10⁻³.Subsequently, TD stretching, or in some case re-stretching is conducted,and after heat treatment, the absolute value of birefringence |Δn| ofthe resulting film is kept below 4×10⁻³ to form a film withwell-balanced strength and linear expansion coefficient.

The temperature for stretching is generally set between glass transitiontemperature of the pre-molded product and melting point of thepre-molded product. The stretching rate is generally 1×10 to 1×10⁵%/min., preferably, 1×10³ to 1×10⁵ %/min. The stretching ratio ispreferably at least 2.5 (MD) and at least 2.0 (TD). Aerial stretchingratio is preferably at least 6 to improve elastic modulus and the like.Too low aerial stretching ratio disadvantageously results in lowdimensional stability.

It is preferable to further conduct heat treatment (annealing or heatsettling) for the stretched film obtained by stretching under theaforementioned conditions when dimensional stability, heat resistance,strength balance of the surface of the film at high temperature arefurther required. Heat treatment may be conducted by the usual method.It can be conducted by maintaining the stretched film in the temperaturerange from glass transition temperature to melting point of the film,preferably, 100° C. lower than melting point to a little lower thanmelting point for 0.5 to 120 seconds under a state of tension, a relaxedstate or a state of controlling shrinkage. Such heat setting may beconducted twice or more changing the conditions within the above range.If the temperature during the heat setting is too low, sufficient heatresistance may not be undesirably obtained during the formation of alayer of magnetic substance by vapor deposition. The heat setting may beconducted in an atmosphere of inert gas such as argon gas, nitrogen gasor the like.

Without the aforementioned heat setting, linear expansion coefficient at0° to 90° C. may become more than 5×10⁻³, and particularly deformationaround glass transition temperature may become greater.

Thus obtained base film for magnetic recording medium, magnetic tape ormagnetic disk of the present invention has linear expansion coefficient,static friction coefficient μs, elastic modulus, surface roughness Raand heat deformation temperature within the above range. This base filmmay be formed into various forms such as tape, disk, card.

As a base film for the magnetic card of the present invention, amonolayer film comprising a composition which is obtained by blending 40to 99% by weight of the aforementioned styrene polymer having a highdegree of syndiotactic configuration and 60 to 1% by weight of a whiteinorganic filler, or a laminate film containing said film may be used.The base film has linear expansion coefficient of not more than 7×10⁻⁵/° C., whiteness of not less than 75, preferably not less than 80, andthickness of 50 to 1,000 μm, preferably 75 to 800 μm. If linearexpansion coefficient of the base film exceeds 7×10⁻⁵ /° C., temperaturechange during reproduction of the record is undesirably remarkable. Ifwhiteness falls below 75, clear characters and picture can not beobtained, that is, it is not preferable in practical application. If thefilm is thinner than 50 μm, strength, durability, maintenance may beinsufficient. On the other hand, too thick film is not preferablebecause processability, handling properties, flexibility are reduced.

More preferably, heat shrinkage ratio of the above base film is not morethan 2% at 200° C. to inhibit deformation during processing and usage.

Considering adhesion between the magnetic layer and the adhesive layerand the like, corona treatment and the like may be conducted onto thesurface of the film.

The above base film can be obtained by a method wherein a compositionobtained by adding a white inorganic filler to a styrene polymer havinga high degree of syndiotactic configuration is formed into a film, or amethod wherein said composition is formed into film on the surface ofthe different film by lamination or coating, or a combination thereof.Most preferably, a composition of a styrene polymer having asyndiotactic configuration added with a white inorganic filler isbiaxially stretched. For addition of such white inorganic filler, amethod by adding at an optional step during polymerization, a method byblending after polymerization, a method by blending master batch or thelike may be employed.

The composition used as a raw material for the base film of the magneticcard of the present invention comprises 40 to 99% by weight of styrenepolymer having a high degree of syndiotactic configuration and 60 to 1%by weight of white inorganic filler. If the amount of the whiteinorganic filler to be compounded exceeds 60% by weight, the resultingmagnetic card has high elastic modulus but is too brittle to be used. Acomposition with the amount of the filler over 60% by weight isdifficult to be stretched and to improve toughness by stretching, and ifit is less than 1% by weight, desirable whiteness for a magnetic cardcan not be shown.

The styrene polymer having a high degree of syndiotactic configurationis as described above.

As the white inorganic filler forming a composition, which forms a rawmaterial of the base film, in combination with the above styrene polymerincludes various kinds, preferably, a compound comprising at least oneelement selected from a group consisting of the group of IIa, IIb, IIIb,IVa, IVb, Vb, VIa, VIIa in a periodic table, for example, magnesiumoxide, aluminum oxide, silicon oxide, titanium oxide, calcium carbonate,barium sulfate, magnesium carbonate, calcium silicate, or a mixturethereof. An average particle diameter of the white filler is notparticularly limited, but preferably, 0.03 to 5 μ. The amount to beadded is as described above. Particularly, in the case of a monolayerfilm, 1 to 40% by weight is preferable, and 3 to 30% by weight is morepreferable. In the case of laminate or coating, 5 to 60% by weight ispreferable and 8 to 55% by weight is more preferable. If the styrenepolymer is whitened during stretching like p-methylstyrene homopolymerto give a white film, the amount to be added may be small, that is, 1 to20% by weight.

The resin components constituting the base film of the presentinvention, in principle, is the aforementioned styrene polymer having asyndiotactic configuration. Considering moldability, mechanicalproperties, surface properties and the like, other resin and the likemay be optionally compounded within the range so long as the object ofthe present invention is not hindered. Such resin includes the sameresins as those shown above. If incompatible resin is used as said otherresin, the amount of white inorganic filler to be added may beconsiderably reduced.

In the present invention, stabilizer, antioxidant, lubricant,brightening agent, UV absorbent, matting agent, or other additives maybe added to the above composition as needed.

In the process for forming the base film of the magnetic card of thepresent invention, the operations from heat melting to heat treatment(annealing) explained for the base film of the above magnetic recordingmedium may be conducted. After operation of annealing, addition ofinorganic or organic filler into the film, lamination of toughened film,sand mat-treatment, coating mat treatment, embossing treatment or thelike may be optionally conducted.

Thus obtained base film of the magnetic card of the present inventionhas linear expansion coefficient of not more than 7×10⁻⁵ /° C.,whiteness of not less than 80, thickness of 50 to 1,000 μm.

By forming at least one magnetic layer is formed onto thus obtained basefilm, the magnetic recording medium, magnetic tape, magnetic disk ormagnetic card of the present invention can be prepared. Further,undercoating layer, back coating layer or topcoating layer may beformed. These layers may be totally or partly formed on both or eitherside of the base film.

The magnetic materials which is a raw material of the magnetic layerincludes various materials, for example, Co, Co--O, Co--Cr, Co--V,Co--Ni, Co--P, Co--γFe₂ O₃, Co--Ni--P, Co--Ni--N, Co--Ni--W, Co--Ni--Pt,CoNi(Cr)/Cr, Fe, Fe--O, Fe--Ag, γFe₂ O₃, Fe--Co, BaO.γFe₂ O₃, Ni, CrO₂.Particularly, for production of densified magnetic recording medium, itis preferable to use a vertically magnetizable material such as Co,Co--O, Co--Cr, Co--V, Co--Ni, Co--P, Co--N--O, CO--Nt--P, Co--Ni--N,Co--Ni--W, CoNi(Cr)/Cr, BaO.γFe₂ O₃ and the like.

A magnetic layer is formed on a base film using this magnetic materialaccording to various methods such as coating, vapor deposition,sputtering, metal plating method or the like. The operation conditionsand the like may be optionally selected according to the conventionalmethods. It is particularly preferable to form Co and BaO.γFe₂ O₃magnetic materials by vapor deposition or sputtering, and coating,respectively, from the viewpoint of formation of vertically magneticlayer.

Thickness of the magnetic layer is not particularly limited, butgenerally 0.01 to 10 μm, and particularly 0.5 to 10 μm in the case ofcoating, 0.01 to 1 μm in the case of vapor deposition and sputtering,and 0.1 to 5 μm in the case of metal plating.

The resin for binder used for coating includes, for example, vinylcopolymer resin such as vinyl chloride--vinyl acetate copolymer,vinyl-chloride--vinyl acetate partly saponified copolymer, vinylchloride--vinylidene chloride copolymer, vinyl chloride--acrylonitrile,vinyl butyral, vinyl formal; fiber resin such as nitrocellulose,cellulose acetobutyrate; condensation polymerization resin such assaturated polyester, polyurethane polyamide, epoxy; synthetic rubberresin such as butadiene acrylonitrile copolymer; inorganicmacromolecular resin such as phosphasene. A crosslinking agent such asisocyanate compound may be used.

The surface of thus obtained magnetic recording medium may be polishedto prevent abrasion of a magnetic head.

The present invention is described in greater detail with reference toexamples, comparative examples and reference examples.

REFERENCE EXAMPLE 1 (Preparation of a Material for Molding of StyrenePolymer containing inorganic filler (a polymer composition)

In a 500-milliliter glass vessel in which air had been replaced withargon, 17.8 g (71 mmol) of copper sulfate pentahydrate (CuSO₄ ·5H₂ O),200 ml of toluene and 24 ml (250 mmol) of trimethylaluminum were placedand reacted at 40° C. for 8 hours. And then, from the solution obtainedby removing the solid, toluene was further distilled away under reducedpressure at room temperature to obtain 6.7 g of a catalytic product. Themolecular weight of the product as determined by the freezing pointdepression method was 610. The aforementioned high magnetic component(i.e., -0.1 to -0.5 ppm) as observed by ¹ H-NMR was 43%.

Separately, 0.4 parts of dry method silica (Aerosil TT-600 (diameter ofprimary particle, 40 μm), manufactured by Degussa) was added to 100parts of pure styrene monomer, and the resultant was mixed and stirredin a cylindrical container using T. K. Homomixer type L (manufacture byTokushukika Kogyo) to prepare a styrene mixture. In this step, 0.05parts by weight of calcium stearate was added.

In a 2-liter reactor, 1 L (L=liter) of styrene mixture obtained by theabove method, 5 mmol of the catalytic product obtained by the abovemethod as aluminum atom, 5 mmol of triisobutyl aluminum and 0.025 mmolof pentamethylcyclopentadienyltitanium trimethoxide were placed andpolymerized at 90° C. for 5 hours. After completion of the reaction, acatalyst component of the product was decomposed with sodium hydroxidein methanol, then the resultant was washed with methanol repeatedly anddried to obtain 308 g of a polymer.

The weight average molecular weight of said polymer as determined by gelpermeation chromatography at 135° C. using 1,2,4-trichlorobenzene as asolvent was 389,000, and weight average molecular weight/number averagemolecular weight was 2.64. The melting point and ¹³ C-NMR measurementsconfirmed that said polymer was polystyrene having a syndiotacticconfiguration.

The polymer was dissolved in 1,2,4-trichlorobenzene at 130° C., thenfiltered and the silica content in the polymer was determined to 0.4% byweight. This solution was dropped onto a slide glass, and observed by amicroscope. As the result, an average particle diameter of silica was0.08 μm.

Further, the styrene polymer was dried in a vacuum at 150° C. for 2hours. The resulting powder was extruded at 300° C. by a vented biaxialextruder equipped with a die containing several capillaries at the tipthereof, then cooled and cut to prepare pellet. The pellet wascrystallized and dried in a hot air with stirring. The pellet hadcrystallinity of 35% and contained 700 ppm of styrene polymer.

(2) Preparation of styrene polymer without containing inorganic filler

In the same manner as that in the above (1) using styrene polymerwithout containing dry method silica, styrene polymer was prepared. Theresulting polymer had weight average molecular weight of 417,000, weightaverage molecular weight/number average molecular weight of 2.54, andcontained 75 ppm of A1 and 2 ppm of Ti.

The styrene polymer was formed into pellet in the same manner as that inthe above (1). Crystallinity and styrene monomer content of the pelletwere 30% and 800 ppm, respectively.

REFERENCE EXAMPLE 2 (Production of Stretched Film of Styrene Polymer)

(1) The material for molding obtained in the same manner as that inReference Example 1 (1) was used and melt extruded by a serial tandemtype extruder with a T-die at the tip thereof at 330° C. Shear stresswas 1.5×10⁵ dyne/cm². The melt extruded sheet was contacted and cooledonto a metal cooling roller at 63° C. by electrostatic application tocool and solidify. In this step, the cooling rate was 55° C./sec in anaverage and 130 μm thick sheet for stretching was obtained. This sheetwas stretched between rollers in MD (by three times) at 110° C. andstretching rate of 6,000%/min while circumferential speed of each rollerwas changed. Subsequently, the sheet was stretched by three times in TDat 120° C. and stretching rate of 6,000%/min using a tenter. Further,the sheet was re-stretched in MD by 1.5 times at 130° C. and 2,000%/minwhile fixed in TD using a tenter. This film was fixed on a tenter,slightly relaxed and heat treated at 255° C. for 10 seconds.

Thickness of the film thus obtained was 12 μm. Linear expansioncoefficient of the film was measured at 0° C. to 90° C. Moreover,surface roughness was measured according to JIS B-0601 (cut-off: 0.08mm), and static friction coefficient was measured according to ASTMD-1984B. The properties of the resulting film are shown in the table.

(2) The operation of Reference Example 2 (1) was repeated, except thatlip opening of the T-die and re-stretching ratio were increased by fourtimes and 1.3 times, respectively. The properties of the resulting filmare shown in the table.

(3) The operation of Reference Example 2 (1) was repeated, except thatthe crystallized pellet of styrene polymers of Reference Example 1 (1)and (2) were melt coextruded by an extruder with a T-die at the tipthereof at 330° C. The properties of the resulting film are shown in thetable. In this case, pellet of styrene polymers of Reference Example 1(2) and (1) were melt-extruded by a serial tandem type uniaxial extruderwith a main extruder containing 50/150/400/150/50 mesh and a doubleextruder, respectively.

(4) The operation of Reference Example 2 (1) was repeated, except thatpellet of styrene polymer of Reference Example 1 (2) was used and50/150/400/150/50 mesh was placed in an extruder. The film was coronatreated. Subsequently, a 0.5 wt % solution of styrene having asyndiotactic configuration--divinyl benzene copolymer obtained inExample 1 of Japanese Patent Application Laid-Open Publication No.95113/1989 (wherein divinyl benzene unit, 9.4 mol %; ethyl benzene unit,5.0 mol %; weight average molecular weight, 360,000) in chloroform wasprepared. Dry method silica (Aerogil TT-600 (diameter of primaryparticle, 40 μm) manufactured by Degussa) (0.5 wt % based onstyrene--divinyl benzene copolymer) was added to the above solution andhomogeneously mixed in a cylindrical container using a homomixer type L(manufactured by Tokushukika Kogyo) to prepare a slurry solution. Theslurry solution was coated on the above film using a bar coater, anddried at 250° C. for 10 seconds. The properties of the resulting filmare shown in the table.

(5) The operation of Reference Example 2 (1) was repeated, except thatheat treatment was not carried out.

(6) The operation of Reference Example 1 (1) was repeated, except thatthe inorganic filler used was silica having an average diameter of 4 μm(Silton AMT-40 manufactured by Mizusawa Kagaku Kogyo), and after astyrene polymer was prepared, the operation of Reference Example 2 (1)was repeated.

(7) In the same manner as that in Reference Example 2 (1), a film wasprepared from a styrene polymer of Reference Example 1 (2).

EXAMPLE 1

Corona treatment was carried out on one side of the base film obtainedin the same manner as that in Reference Example 2 (1). Subsequently, atarget comprising 80 wt % Co and 20 wt % Ni was prepared, and a Co--Nimagnetic thin layer about 3,000 A thick was formed using this target byRF sputter method.

The distance between the target and the film was 60 mm, plate voltagewas 1.9 kV, plate current was 160 mA and argon pressure was 1.1×10⁻²mmHg.

The tape was carefully slitted to the same width as that of VHS videotape. The commercially available tape was dismantled, only tape wasreplaced and video cassette was prepared.

Sliding properties were estimated by static friction coefficient of thistape. Static friction coefficient of this tape was as good as that ofthe base film.

The prepared VHS video cassette tape was used for record andreproduction by a home video recorder. As an index of reliability, theconditions of record and reproduction were observed under roomtemperature conditions and under high temperature and high humidityconditions (85° C., 75% RH). Little difference was observed between bothcase. Both cases were good, presenting no disturbed picture. The resultsare shown in Table 1.

EXAMPLE 2

One side of the base film obtained in the same manner as that inReference Example 2 (1) was corona treated and coated with magneticcoating composition. The constitution of this magnetic coatingcomposition was as follows: γ-FeO₃ magnetic powder, 45 parts by weight;vinyl chloride--vinyl acetate copolymer (VAGH manufactured by U.C.C.),17 parts by weight; acrylonitrile--butadiene copolymer (N 1432 Jmanufactured by Nippon Zeon), 3.5 parts by weight; polyisocyanate(coronate L manufactured by Nippon polyurethane), 1.5 parts by weight,methyl isobutyl ketone, 50 parts by weight; toluene, 50 parts by weightand carbon black, 4 parts by weight. Thickness of the magnetic layerafter drying was 3 μm.

The procedure of Example 1 was repeated using this tape. The results areshown in Table 1.

EXAMPLE 3

After corona discharge treatment was carried out on the both sides ofthe base film obtained in the same manner as that in Reference Example 2(2), magnetic layers were formed on the both sides in the same manner asthat in Example 1. The both sides of this film were properly polished,then cut into the same size as the commercially available floppy(5-inch). Further, this disc was enclosed in a jacket of thecommercially available floppy to prepare a floppy disc. Static frictioncoefficient of the film with a magnetic layer was determined in the samemanner as that in Example 1. The conditions of SAVE and LOAD wereobserved using the prepared floppy and NEC PC-9801F under roomtemperature conditions and high temperature and high humidityconditions. Both case was good. The results are shown in Table 1.

EXAMPLE 4

The procedure of Example 1 was repeated, except that a magnetic layerwas formed on the side of smooth surface of the film obtained in thesame manner as that in Reference Example 2 (3). The results are shown inTable 1.

EXAMPLE 5

The procedure of Example 1 was repeated, except that a magnetic layerwas formed on the side of smooth surface of the film obtained in thesame manner as that in Reference Example 2 (4). The results are shown inTable 1.

COMPARATIVE EXAMPLE 1

The procedure of Example 1 was repeated, except that a magnetic layerwas formed on one side of the film obtained in the same manner as thatin Reference Example 2 (5). The results are shown in Table 1.

COMPARATIVE EXAMPLE 2

The procedure of Example 1 was repeated, except that a magnetic layerwas formed on one side of the film obtained in the same manner as thatin Reference Example 2 (6). The results are shown in Table 1.

COMPARATIVE EXAMPLE 3

The procedure of Example 1 was repeated, except that a magnetic layerwas formed on one side of the film obtained in the same manner as thatin Reference Example 2 (7). The results are shown in Table 1.

COMPARATIVE EXAMPLE 4

The procedure of example 1 was repeated, except that polyethyleneterephthalate (PET) film (Tetron NS 12 μm, manufactured by Teijin) wasused. The results are shown in Table 1.

                                      TABLE 1                                     __________________________________________________________________________    Stretched Film of Styrene Polymer                                                                Linear                                                           Production   Expansion                                                                            Surface                                                   Process                                                                              Thickness                                                                           Coefficient.sup.1)                                                                   Roughness Ra.sup.2)                                                                   Static                                      No.   No.    (μm)                                                                             (°C..sup.-1)                                                                  (μm) Friction Coefficient.sup.3)                 __________________________________________________________________________    Example 1                                                                           Ref. Ex. 2(1)                                                                        12    3 × 10.sup.-5                                                                  0.013   0.54                                        Example 2                                                                           Ref. Ex. 2(1)                                                                        12    3 × 10.sup.-5                                                                  0.013   0.54                                        Example 3                                                                           Ref. Ex. 2(2)                                                                        75    4 × 10.sup.-5                                                                  0.014   0.49                                        Example 4                                                                           Ref. Ex. 2(3)                                                                        12    2 × 10.sup.-5                                                                  0.008/0.014                                                                           0.58                                        Example 5                                                                           Ref. Ex. 2(4)                                                                        12    2 × 10.sup.-5                                                                  0.006/0.022                                                                           0.39                                        Com. Ex. 1                                                                          Ref. Ex. 2(5)                                                                        12    1 × 10.sup.-4                                                                  0.014   0.53                                        Com. Ex. 2                                                                          Ref. Ex. 2(6)                                                                        12    3 × 10.sup.-5                                                                  0.056   0.85                                        Com. Ex. 3                                                                          Ref. Ex. 2(7)                                                                        12    2 × 10.sup.-5                                                                  0.005   1.12                                        Com. Ex. 4                                                                          PET    12    2 × 10.sup.-5                                                                  0.02    0.45                                        __________________________________________________________________________    Magnetic Recording Medium                                                                Thickness                                                                     of Magnetic      ecording and Reproduction.sup. 5)                       Magnetic                                                                           Layer      Sliding                                                                             Room   High Temperature                           No.   Material                                                                           (μm)                                                                              Shape                                                                             Properties.sup.4)                                                                   Temperature                                                                          High Humidity                              __________________________________________________________________________    Example 1                                                                           Co--Ni                                                                             0.3    Tape                                                                              Good  Good   Good                                       Example 2                                                                           γ-Fe.sub.2 O.sub.3                                                           3      Tape                                                                              Good  Good   Good                                       Example 3                                                                           Co--Ni                                                                             0.3    Disk                                                                              Good  Good   Good                                       Example 4                                                                           Co--Ni                                                                             0.3    Tape                                                                              Good  Good   Good                                       Example 5                                                                           Co--Ni                                                                             0.3    Tape                                                                              Good  Good   Good                                       Com. Ex. 1                                                                          Co--Ni                                                                             0.3    Tape                                                                              Good  Good   Bad                                        Com. Ex. 2                                                                          CO--NI                                                                             0.3    Tape                                                                              Good  Bad    --                                         Com. Ex. 3                                                                          Co--Ni                                                                             0.3    Tape                                                                              Bad   Bad    --                                         Com. Ex. 4                                                                          Co--Ni                                                                             0.3    Tape                                                                              Good  Good   Bad                                        __________________________________________________________________________     .sup.1) Measured at 0 to 90° C.                                        .sup.2) According to JIS B0601, cutoff: 0.08 mm                               .sup.3) According to ASTM D1984                                               .sup.4) Static friction coefficient: 0.03-1.0: good, ≧1: bad           .sup.5) Image recorded on a tape was reproduced and observed.                 Good: without disturbance of picture, Bad: with disturbance of picture        (high temperature and high humidity means 85° C. and 75 RH)       

REFERENCE EXAMPLE 3 (Production of Stretched Film of Styrene Polymer)

(1) The material for molding obtained in the same manner as that inReference Example 1 (1) was used and melt extruded by a serial tandemtype extruder with a T-die at the tip thereof at 330° C. Shear stresswas 1.5×10⁵ dyne/cm². The melt extruded sheet was contacted and cooledonto a metal cooling roller at 63° C. by electrostatic application tocool and solidify. In this step, the cooling rate was 55° C./sec in anaverage and 130 μm thick sheet for stretching was obtained. This sheetwas stretched between rollers in MD (by three times) at 110° C. andstretching rate of 6,000%/min while circumferential speed of each rollerwas changed. Subsequently, the sheet was stretched in TD at 120° C. and6,000%/min by 3 times using a tenter. Further, the sheet was re-strechedin MD at 130° C. and 2,000%/min by 1.5 times while fixed in TD using atenter. This film was fixed on a tenter, slightly relaxed and heattreated at 255° C. for 10 seconds.

Thickness of the film thus obtained was 12 μm. Moisture expansioncoefficient of the film was measured in the range from 20 to 80% RH.Expansion coefficient was measured at 0° C. to 90° C. Moreover, surfaceroughness was measured according to JIS B-0602 (cut-off: 0.08 mm), andstatic friction coefficient was measured according to ASTM D-1984B. Heatdeformation temperature of this film was measured by thermal mechanicalanalysis (TMA). Heat deformation temperature means the temperature atwhich 2% change of the specimen length was observed. The properties ofthe resulting film are shown in Table 2.

(2) The procedure of Reference Example 3 (1) was repeated, except thatlip opening of the T-die and re-stretching ratio were increased by 4times and 1.3 times, respectively.

(3) The procedure of Reference Example 3 (1) was repeated, except thatthe crystallized pellet of styrene polymers of Reference Example 1 (1)and (2) were melt coextruded by an extruder with a T-die at the tipthereof at 330° C. The properties of the resulting film are shown inTable 2. In this case, pellet of styrene polymers of Reference Example 1(2) and (1) were melt extruded by a serial tandem type uniaxial extruderwith a main extruder containing 50/150/400/150/50 mesh and a doubleextruder, respectively.

(4) The procedure of Reference Example 3 (1) was repeated except thatpellet of styrene polymer of Reference Example 1 (2) was used and50/150/400/150/50 mesh was placed in an extruder. The film was coronatreated. Subsequently, a 0.5 wt % solution of styrene having asyndiotactic configuration-divinyl benzene copolymer obtained in Example1 of Japanese Patent Application Laid-Open Publication No. 95113/1989(wherein divinyl benzene unit, 9.4 mol %; ethyl benzene unit, 5.0 mol %;weight average molecular weight, 360,000) in chloroform was prepared.Dry method silica (Aerogil TT-600 (diameter of primary particle, 40 μm)manufactured by Degussa) (0.5 wt % based on styrene--divinyl benzenecopolymer) was added to the above solution and homogeneously mixed in acylindrical container using a homomixer type L (manufactured byTokushukika Kogyo) to prepare a slurry solution. The slurry solution wascoated on the above film using a bar coater, and dried at 250° C. for 10seconds. The properties of the resulting film are shown in Table 2.

(5) The procedure of Reference Example 3 (1) was repeated, except thatheat treatment was not carried out.

(6) In the same manner as that in Reference Example 3 (1), a film wasprepared from a styrene polymer of Reference Example 1 (2).

EXAMPLE 6

Corona treatment was carried out on one side of the base film obtainedin the same manner as that in reference Example 3 (1). Subsequently, a0.1 μm thick Co--Cr membrane (Cr: 20 wt %) was formed by a vacuum vapordeposition. In this case the temperature of the base was 250° C., andcurling of the tape or the like was not observed. Static frictioncoefficient of the tape after such vapor deposition was determinedaccording to ASTM D-1984 and was 0.55, and the tape presented goodsliding properties. Coersive force after vapor deposition was 900 Oe, asdetermined from a magnetizing characteristic curve using a vibrationsample magnetometer VSM. The results are shown in Table 2.

EXAMPLE 7

One side of the film of Reference Example 3 (1) was corona treated andcoated with magnetic coating composition. The constitution of thismagnetic coating composition was as follows: barium ferrite magneticpowder, 100 parts by weight; vinyl chloride--vinyl acetate copolymer(VAGH manufactured by U.C.C.), 15 parts by weight; polyurethane, 20parts by weight; lecitin, 4 parts by weight; stearic acid, 3 parts byweight; methyl isobutyl ketone, 100 parts by weight; methyl ethylketone, 100 parts by weight; cyclohexanone, 100 parts by weight; andisocyanate, 3 parts by weight.

Thickness of the magnetic layer after drying was 3 μm. This magnetictape was examined in the same manner as in Example 6. The results areshown in Table 2.

EXAMPLE 8

The procedure of Example 6 was repeated, except that both sides of thefilm of Reference Example 3 (2) were corona treated and magneticmaterials were coated thereon, and the film was formed into a disc. Theresults are shown in Table 2.

EXAMPLE 9

The one side of the film of Reference Example 3 (3) was corona treatedand on which a 0.8 μm thick Co--O membrane (0.21 wt %) was formed.During the formation of this membrane, electron beam was used forevaporation of Co, O₂ gas was introduced and separation speed was about900Å/sec. O₂ composition in the membrane was analyzed using EPMA. Thevapor-deposited tape was investigated in the same manner as that inExample 6. The results are shown in Table 2.

EXAMPLE 10

The procedure of Example 9 was repeated, except that Co--Ni--O membrane(0.22 wt %) was formed using CoNi (Ni, 0.1 wt %). The results are shownin Table 2.

COMPARATIVE EXAMPLE 5

The procedure of Example 6 was repeated, except that a film of ReferenceExample 3 (5) was used and temperature of the base was set at 100° C.The results are shown in the table. Formation of a magnetic layer at thebase temperature of 250° C. was difficult due to shrinkage and curlingof the film. The results are shown in Table 2.

COMPARATIVE EXAMPLE 6

The procedure of Example 6 was repeated, except that a film of ReferenceExample 3 (6) was used. The results are shown in Table 2.

COMPARATIVE EXAMPLE 4

The procedure of Example 6 was repeated, wherein PET film (Tetron NS 12μm, manufactured by Teijin) was used and the temperature of the base was200° C. The results are shown in the table. Formation of the magneticlayer at temperature of the base of 250° C. was difficult because filmwas partly melted, stretched or the like. The results are shown in Table2.

                                      TABLE 2                                     __________________________________________________________________________           Stretched Film of styrene Polymer                                                       Linear                Heart                                         Production                                                                          Thick-                                                                            Expansion                                                                            Surface Static Deformation                                   Process                                                                             ness                                                                              Coefficient.sup.1)                                                                   Roughness Re.sup.2)                                                                   Friction                                                                             Temperature                            No.    No.   (μm)                                                                           (°C..sup.-1)                                                                  (μm) Coefficient.sup.3)                                                                   (°C.)                           __________________________________________________________________________    Example 6                                                                            Ref. Ex. 3(1)                                                                       12  3 × 10.sup.-5                                                                  0.013   0.54   255                                    Example 7                                                                            Ref. Ex. 3(1)                                                                       12  3 × 10.sup.-5                                                                  0.013   0.54   255                                    Example 8                                                                            Ref. Ex. 3(2)                                                                       75  4 × 10.sup.-5                                                                  0.014   0.49   257                                    Example 9                                                                            Ref. Ex. 3(3)                                                                       12  2 × 10.sup.-5                                                                  0.008/0.014                                                                           0.58   253                                     Example 10                                                                          Ref. Ex. 3(4)                                                                       12  2 × 10.sup.-5                                                                  0.006/0.022                                                                           0.39   254                                    Com. Ex. 5                                                                           Ref. Ex. 3(5)                                                                       12  1 × 10.sup.-4                                                                  0.014   0.53   110                                    Com. Ex. 6                                                                           Ref. Ex. 3(6)                                                                       12  2 × 10.sup.-5                                                                  0.005   1.12   254                                    Com. Ex. 7                                                                           PET   12  2 × 10.sup.-5                                                                  0.02    0.45   230                                    __________________________________________________________________________           Magnetic Recording Medium                                                            Thickness                                                                     of Magnetic      Coersive                                              Magnetic                                                                             Layer      Sliding                                                                             Force Hc.sup.5)                                                                     Conditions after                         No.    Material                                                                             (μm)                                                                              Shape                                                                             Properties.sup.4)                                                                   (Oe)  metallizing *6                           __________________________________________________________________________    Example 6                                                                            CO--CR 0.1    Tape                                                                              Good  900   Good                                     Example 7                                                                            BaO--Fe.sub.2 O.sub.3                                                                3      Tape                                                                              Good  720   Good                                     Example 8                                                                            Co--Cr 0.1    Disk                                                                              Good  910   Good                                     Example 9                                                                            Co--O  0.8    Tape                                                                              Good  1,100 Good                                      Example 10                                                                          Co--Ni--O                                                                            0.8    Tape                                                                              Good  800   Good                                     Com. Ex. 5                                                                           Co--Cr 0.1    Tape                                                                              Good  420   Bad                                      Com. Ex. 6                                                                           Co--Cr 0.1    Tape                                                                              Bad   880   --                                       Com. Ex. 7                                                                           Co--Cr 0.1    Tape                                                                              Good  620   Bad                                      __________________________________________________________________________     .sup.1) Measured at 0 to 90° C.                                        .sup.2) According to JIS B0601, cutoff: 0.08 mm                               .sup.3) According to ASTM D1984                                               .sup.4) Static friction coefficient: 0.03-1.0: good, ≧1: bad           .sup.5) Determined from magnetization characteristic curve by vibration       sample type magnetometer VSM                                                  *6: with curling: bad, without curling: good                             

REFERENCE EXAMPLE 4 (Production of Stretched Film of Styrene Polymer)

(1) The material for molding obtained in the same manner as that inReference Example 1 (1) was used and melt extruded by a serial tandemtype extruder with a T-die at the tip thereof at 330° C. Shear stresswas 1.5×10⁵ dyne/cm². The melt extruded sheet was contacted onto acooling roller at 70° C. by electrostatic application to cool andsolidify. In this step, the cooling rate was 60° C./sec in an averageand 110 μm thick sheet for stretching was obtained. This sheet wasstretched between rollers in MD (by three times) at 110° C. andstretching rate of 6,000%/min while circumferential speed of each rollerwas changed. Subsequently, the sheet was stretched in TD by three timesat 120° C. and stretching rate of 6,000%/min. Further, the sheet wasre-stretched in Md at 130° C. and 2,000%/min by 1.5 times while fixed inTD using a tenter. This film was fixed on a tenter, slightly relaxed andheat treated at 255° C. for 10 seconds.

Thickness of the film thus obtained was 8 μm. Elastic moduli of the filmin MD and TD at room temperature and elastic modulus in MD at 90° C.were measured. Linear expansion coefficient of the film at 0° C. to 90°C. was measured by thermal mechanical analysis. Moreover, surfaceroughness was measured according to JIS B-060l (cut-off: 0.08 mm), andstatic friction coefficient was measured according to ASTM D-1984B. Theproperties of the resulting film are shown in Table 3.

(2) The procedure of Reference Example 4 (1) was repeated, except thatstretching ratio of the film in MD and in TD was increased by 3.5 timesand 2.7 times, respectively. The properties of the resulting film areshown in Table 3.

(3) The procedure of Reference Example 4 (1) was repeated, except thatthe crystallized pellets of styrene polymers of Reference Example 1 (1)and (2) were melt coextruded by an extruder with a T-die at the tipthereof at 330° C. The properties of the resulting film are shown inTable 3. In this case, pellets of styrene polymers of Reference Example1 (2) and (1) were melt-extruded by a serial tandem type uniaxialextruder with a main extruder containing 50/150/400/150/50 mesh and adouble extruder, respectively.

(4) The procedure of Reference Example 4 (1) was repeated, except thatpellet of styrene polymer of Reference Example 1 (2) was used and50/150/400/150/50 mesh was placed in an extruder. The film was coronatreated. Subsequently, a 0.5 wt % solution of styrene having asyndiotactic configuration--divinyl benzene copolymer obtained inExample 1 of Japanese Patent Application Laid-Open Publication No.95113/1989 (wherein divinyl benzene unit, 9.4 mol %; ethyl benzene unit,5.0 mol %; weight average molecular weight, 360,000) in chloroform wasprepared. Dry method silica (Aerogil TT-600 (diameter of primaryparticle, 40 μm) manufactured by Degussa) (0.5 wt % based onstyrene--divinyl benzene copolymer) was added to the above solution andmixed homogeneously in a cylindrical container using a homomixer type L(manufactured by Tokushukika Kogyo) to prepare a slurry solution. Theslurring solution was coated on the above film using a bar coater, anddried at 250° C. for 10 seconds. The properties of the resulting filmare shown in Table 3.

(5) The procedure of Reference Example 4 (1) was repeated, except thatthe stretching ratio in MD and that in TD were doubled and restretchingwas not carried out. The properties of the resulting film are shown inTable 3.

(6) The procedure of Reference Example 4 (1) was repeated, except thatthe stretching ratio in MD was increased by 4 times and restretching wasnot carried out. The properties of the resulting film are shown in Table3.

(7) The procedure of Reference Example 4 (1) was repeated, except thatthe stretching ratio in MD and that in TD were increased by 4 times andtwice, respectively and restretching was not carried out. The propertiesof the resulting film are shown in Table 3.

(8) The procedure of Reference Example 4 (1) was repeated, except thatheat -treatment was not carried out. The properties of the resultingfilm are shown in Table 3.

(9) The procedure of Reference Example 1 (1) was repeated, except that 4μm silica (Silton AMT-40 manufactured by Mizusawa Kagaku Kogyo) was usedto prepare a styrene polymer, and subsequent operation was carried outin the same manner as that in Reference Example 4 (2). The properties ofthe resulting film are shown in Table 3.

EXAMPLE 11

Corona treatment was carried out on one side of the base film obtainedin Reference Example 4 (1). Subsequently, a target comprising 80 wt % Coand 20 wt % Ni was prepared, and a Co--Ni magnetic thin layer about3,000Å thick was formed using this target by RF sputter method.

The distance between the target and the film was 60 mm, plate voltagewas 1.9 kV, plate current was 160 mA and argon pressure was 1.1×10⁻²mmHg.

The tape was carefully slitted to the same width as that of audio tape.The commercially available cassette tape was dismantled, only tape wasreplaced and audio cassette was prepared.

Sliding properties of this tape were evaluated by static frictioncoefficient according to ASTM D-1984, which showed a good result of0.55. This tape was recorded and set in a car stereo, then the tape wasrepeatedly played back for 2 hours while the cab temperature wasmaintained at 90° C., and change in musical intervals was checked. Asthe result, the tape was good, showing little change. Further, afterthis tape was heated an cooled between room temperature and 90° C. atthe rate of 10 ° C./min under load of 6 kg/cm², displacement as measuredby thermal mechanical analysis (TMA) was good, i.e. lower than 0.1%. Theresults are shown in Table 3.

EXAMPLE 12

One side of the base film obtained in the same manner as that inReference Example 4 (1) was corona treated and coated with magneticcoating composition. The constitution of this magnetic coatingcomposition was as follows: γ-FeO₃ magnetic powder, 45 parts by weight;vinyl chloride--vinyl acetate copolymer (VAGH manufactured by U.C.C.),17 parts by weight; acrylonitrile--butadiene copolymer (N 1432 Jmanufactured by Nippon Zeon), 3.5 parts by weight; polyisocyanate(coronate L manufactured by Nippon polyurethane), 1.5 parts by weight,methyl isobutyl ketone, 50 parts by weight; toluene, 50 parts by weightand carbon black, 4 parts by weight. Thickness of the magnetic layerafter drying was 3 μm.

The procedure of Example 11 was repeated using this tape. The resultsare shown in Table 3.

EXAMPLE 13

After corona discharge treatment was carried out on one side of the filmobtained in Reference Example 4 (2), 0.1 μm thick Co--Cr membrane (Cr,20 wt %) was formed on the side. In this step, temperature of the basewas 250° C. The results are shown in Table 3.

EXAMPLE 14

The side of smooth surface of the film of Reference Example 4 (3) wascorona treated and a magnetic coating composition was applied on thesurface.

The constitution of this magnetic coating composition was as follows:barium ferrite magnetic powder, 100 parts by weight; vinylchloride--vinyl acetate copolymer (VAGH manufactured by U.C.C.), 15parts by weight; polyurethane, 20 parts by weight; lecitin, 4 parts byweight; stearic acid, parts by weight; methyl isobutyl ketone, 100 partsby weight; methyl ethyl ketone, 100 parts by weight; cyclohexanone, 100parts by weight; and isocyanate, 3 parts by weight. Thickness of themagnetic layer after drying was 3 μm. This tape was investigated in thesame manner as that in Example 11. The results are shown in Table 3.

EXAMPLE 15

The procedure of Example 11 was repeated, except that a magnetic layerwas formed on the side of smooth surface of the film obtained inReference Example 4 (4). The results are shown in Table 3.

COMPARATIVE EXAMPLE 8

The procedure of Example 11 was repeated using the film obtained inReference Example 4 (5). The results are shown in Table 3.

COMPARATIVE EXAMPLE 9

The procedure of Example 11 was repeated using the film obtained in thesame manner as that in Reference Example 4 (6). The results are shown inTable 3.

COMPARATIVE EXAMPLE 10

The procedure of Example 11 was repeated using the film obtained inReference Example 4 (7). The results are shown in Table 3.

COMPARATIVE EXAMPLE 11

The procedure of Example 11 was repeated using the film obtained inReference Example 4 (8). The results are shown in Table 3.

COMPARATIVE EXAMPLE 12

The procedure of Example 11 was repeated using the film obtained inReference Example 4 (9). The results are shown in Table 3.

COMPARATIVE EXAMPLE 13

The procedure of Example 11 was repeated, except that PET film (TetronFilm, 6 μm, manufactured by Teijin) was used. The results are shown inTable 3.

                                      TABLE 3                                     __________________________________________________________________________           Stretched Film of Styrene Polymer                                                                         Linear.sup.2)                                     Production                                                                           Thick-                                                                            Elastic Modulus (kg/cm.sup.2).sup.1)                                                           Expansion                                                                           Surface                                     Process                                                                              ness                                                                              MD  MD  TD       Coefficient                                                                         Roughness Ra.sup.3)                  No.    No.    (μm)                                                                           (RT)                                                                              (90)                                                                              (RT)                                                                              MD/TD                                                                              (°C..sup.-1)                                                                 (μm)                              __________________________________________________________________________    Example 11                                                                           Ref. Ex. 4(1)                                                                        8   45,000                                                                            42,000                                                                            35,000                                                                            1.29 3 × 10.sup.-5                                                                 0.013                                Example 12                                                                           Ref. Ex. 4(1)                                                                        8   45,000                                                                            42,000                                                                            35,000                                                                            1.29 3 × 10.sup.-5                                                                 0.013                                Example 13                                                                           Ref. Ex. 4(2)                                                                        12  34,000                                                                            32,000                                                                            40,000                                                                            0.85 4 × 10.sup.-5                                                                 0.014                                Example 14                                                                           Ref. Ex. 4(3)                                                                        8   44,000                                                                            42,000                                                                            36,000                                                                            1.22 2 × 10.sup.-5                                                                 0.008/0.014                          Example 15                                                                           Ref. Ex. 4(4)                                                                        9   42,000                                                                            40,000                                                                            34,000                                                                            1.23 2 × 10.sup.-5                                                                 0.006/0.022                          Com. Ex. 8                                                                           Ref. Ex. 4(5)                                                                        8   29,000                                                                            28,000                                                                            29,000                                                                            1.0  4 × 10.sup.-5                                                                 0.011                                Com. Ex. 9                                                                           Ref. Ex. 4(6)                                                                        8   50,000                                                                            47,000                                                                            27,000                                                                            1.85 3 × 10.sup.-5                                                                 0.023                                Com. Ex. 10                                                                          Ref. Ex. 4(7)                                                                        8   32,000                                                                            30,000                                                                            45,000                                                                            0.71 2 × 10.sup.-5                                                                 0.017                                Com. Ex. 11                                                                          Ref. Ex. 4(8)                                                                        8   44,000                                                                            42,000                                                                            33,000                                                                            1.33 1 × 10.sup.-4                                                                 0.014                                Com. Ex. 12                                                                          Ref. Ex. 4(9)                                                                        8   46,000                                                                            45,000                                                                            34,000                                                                            1.35 3 × 10.sup.-5                                                                 0.056                                Com. Ex. 13                                                                          PET    6   46,000                                                                            --  49,000                                                                            0.93 2 × 10.sup.-5                                                                 0.02                                 __________________________________________________________________________           Magnetic Recording Medium                                                            Thickness                                                              Magnetic                                                                             of Magnetic Layer                                                                       Sliding                                                                             Recording                                       No.    Material                                                                             (μm)   Properties.sup.4)                                                                   and Reproduction.sup.5)                                                                 Elongation of Tape                    __________________________________________________________________________    Example 11                                                                           Co--Ni 0.3       Good  Good      Good                                  Example 12                                                                           γ-Fe.sub.2 O.sub.3                                                             3         Good  Good      Good                                  Example 13                                                                           Co--Cr 0.1       Good  Good      Good                                  Example 14                                                                           BaO--Fe.sub.2 O.sub.3                                                                3         Good  Good      Good                                  Example 15                                                                           Co--Ni 0.3       Good  Good      Good                                  Com. Ex. 8                                                                           Co--Ni 0.3       Good  Bad       Bad                                   Com. Ex. 9                                                                           Co--Ni 0.3       Good  Bad       Good                                  Com. Ex. 10                                                                          Co--Ni 0.3       Good  Bad       Good                                  Com. Ex. 11                                                                          Co--Ni 0.3       Good  Bad       Bad                                   Com. Ex. 12                                                                          Co--Ni 0.3       Good  Bad       Good                                  Com. Ex. 13                                                                          Co--Ni 0.3       Good  Bad       Bad                                   __________________________________________________________________________     .sup.1) (RT) means room temperature and (90) means 90° C.              .sup.2) Measured at 0 to 90° C.                                        .sup.3) According to JIS B0601, cutoff: 0.08 mm                               .sup.4) According to ASTM D1984 Static friction coefficient: 0.03-1.0:        good, ≧1: bad                                                          .sup.5) Sound recorded the tape was reproduced and observed in a car at       90° C. for 2 hours.                                                    Good: without disturbance of musical interval                                 Bad: with disturbance in musical disturbance                                  .sup.6) Displacement after heating and cooling between room temperature       and 90° C. at a rate of 10° C./min. under load of 6             kg/cm.sup.2 was measured by thermal mechanical analysis (TMA)                 Good: displacement less than 0.1%                                             Bad: displacement not less than 0.1%                                     

REFERENCE EXAMPLE 5 (Production of Stretched Film of Styrene Polymer)

(1) The material for molding obtained in the same manner as that inReference Example 1 (1) was used and melt-extruded by a serial tandemtype extruder with a T-die at the tip thereof at 330° C. Shear stresswas 1.5×10⁵ dyne/cm². The melt extruded sheet was contacted onto a metalcooling roller at 63° C. by electrostatic application to cool andsolidify. In this step, the cooling rate was 70 ° C./sec on an averageand 1,500 μm thick sheet for stretching was obtained- This sheet wasstretched between rollers in MD by three times at 110° C. and stretchingrate of 6,000%/min while circumferential speed of each roller waschanged. Absolute value of birefringence |Δn| was 35×10⁻³.

Subsequently, the sheet was stretched by three times in TD at 120° C.and stretching rate of 6,000%/min using a tenter. Further, the sheet wasre-stretched simultaneously in MD and TD (1.5 times, each) at 130° C.and 2,000%/min while fixed in TD using a tenter. This film was fixed ona tenter, slightly relaxed and heat treated at 255° C. for 10 seconds.

Thickness of the film thus obtained was 75 μm. Absolute value ofbirefringence |Δn| was 14×10⁻³. Linear expansion coefficient of the filmat 0° C. to 90° C. was measured by thermal mechanical analysis.Moreover, surfaces roughness was measured according to JIS B-0601(cut-off: 0.08 mm). The properties of the resulting film are shown inTable 4.

(2) The procedure in Reference Example 5 (1) was repeated, except thatpellets of crystallized styrene polymers of Reference Example 1 (1) and(2) were used and melt-coextruded at 330° C. by an extruder with a T-dieat the tip thereof into three layers of two kinds of polymers. Theproperties of the resulting film are shown in Table 4. In this case, thepellets of styrene polymers of Reference Example 1 (2) and (1) weremelt-extruded by a serial tandem type uniaxial extruder with a mainextruder containing 50/150/400/150/50 mesh and a double extruder,respectively, to give a constitution of polymer of Reference Example 1(1)/polymer of Reference Example 1 (2)/polymer of Reference Example 1(1). Absolute value of birefringence |Δn| of this film was 10×10⁻³.

(3) The procedure in Reference Example 5 (1) was repeated, except thatpellet of styrene polymer of Reference Example 1 (2) was used and50/150/400/150/50 mesh was placed in an extruder. Absolute value ofbirefringence |Δn| of this film was 9×10⁻³. The both sides of the filmwere corona treated.

Subsequently, a 0.5 wt % solution of styrene having a syndiotacticconfiguration--divinyl benzene copolymer obtained in Example 1 ofJapanese Patent Application Laid-Open Publication No. 95113/1989(wherein divinyl benzene unit, 9.4 mol %; ethyl benzene unit, 5.0 mol %;weight average molecular weight, 360,000) in chloroform was prepared.Dry method silica (Aerogil TT-600 (diameter of primary particle, 40 μm)manufactured by Degussa Co.) (0.5 wt % based on styrene --divinylbenzene copolymer) was added to the above solution and homogeneouslymixed in a cylindrical container using a homomixer type L (manufacturedby Tokushukika Kogyo) to prepare a slurry solution. The slurry solutionwas coated on the both sides of the above film using a bar coater, anddried at 250° C. for 10 seconds. The properties of the resulting filmare shown in Table 4.

(4) The procedure in Reference Example 5 (1) was repeated, except thatheat treatment was not: carried out. Absolute value of birefringence|Δn| of this film was 18×10¹⁰ ⁻³. The properties of the resulting filmare shown in Table 4.

(5) The procedure in Reference Example 5 (1) was repeated, except thatre-stretching ratio in MD was changed to 2. Absolute value ofbirefringence |Δn| of this film was 53×10⁻³. The properties of theresulting film are shown in Table 4.

(6) The procedure in Reference Example 1 (1) was repeated, except thatthe inorganic filler used was silica having an average diameter of 4 μm(Silton AMT-40 manufactured by Mizusawa Kagaku Kogyo), and after astyrene polymer was prepared, the procedure in Reference Example 5 (1)was repeated. Absolute value of birefringence |Δn| of this film was8×10⁻³. The properties of the resulting film are shown in Table 4.

EXAMPLE 16

Both sides of the base film obtained in the same manner as that inReference Example 2 (1) were corona treated and coated with magneticcoating composition. The constitution of this magnetic coatingcomposition was as follows: γ-Fe₂ O₃ magnetic powder, 45 parts byweight; vinyl chloride--vinyl acetate copolymer (VAGH manufactured byU.C.C.), 17 parts by weight; acrylonitrile--butadiene copolymer (N 1432J manufactured by Nippon Zeon), 3,5 parts by weight; polyisocyanate(coronate L manufactured by Nippon polyurethane), 1.5 parts by weight,methyl isobutyl ketone, 50 parts by weight; toluene, 50 parts by weightand carbon black, 4 parts by weight. Thickness of the magnetic layerafter drying was 3 μm.

Subsequently, the both sides of this film were properly polished, thencut into the same size as the commercially available floppy (5-inch).Further, this disk was enclosed in a Jacket of the commerciallyavailable floppy disk to prepare a floppy disk. A program was fed tothis floppy using NEC PC-9801F. Then, the floppy was left at 80° C.under 70% RH (relative humidity) for 50 hours. After the floppy wasbrought to the normal condition and LOAD and RUN of the program werecarried out. The conditions were good. This fact showed that dimensionalchanges in all directions under high temperature and high humidityconditions were very small and well balanced. LOAD and RUN werecontinuously repeated (×100), resulting in no special abnormality butgood sliding properties and good performance traverse. The results areshown in Table 4.

EXAMPLE 17

Corona treatment was carried out on both sides of the base film obtainedin the same manner as that in Reference Example 5 (1). Subsequently, atarget comprising 80 wt % Co and 20 wt % Ni was prepared. and a Co--Nimagnetic thin films of about 3,000Å thickness were formed on both sidesusing this target by RF sputter method, which was subjected to anodicoxidation in a solution containing an electrolyte to form passive statemembrane on the surface. As condition for sputtering, the distancebetween the target and the film was 60 mm, plate voltage was 1.9 kV,plate current was 160 mA and argon pressure was 1.1×10⁻² mmHg.

The procedure in Example 16 was repeated using this film. The resultsare shown in Table 4.

EXAMPLE 18

The procedure in Example 16 was repeated, except that the film obtainedin Reference Example 5 (1) was used. The results are shown in Table 4.

EXAMPLE 19

The procedure in Example 16 was repeated, except that the film obtainedin Reference Example 5 (3) was used. The results are shown in Table 4.

COMPARATIVE EXAMPLE 14

The procedure in Example 16 was repeated, except that the film obtainedin Reference Example 5 (4) was used. The results are shown in Table 4.

COMPARATIVE EXAMPLE 15

The procedure in Example 16 was repeated, except that the film obtainedin Reference Example 5 (5) was used. The results are shown in Table 4.

COMPARATIVE EXAMPLE 16

The procedure in Example 16 was repeated, except that the film obtainedin Reference Example 5 (6) was used. The results are shown in Table 4.

COMPARATIVE EXAMPLE 17

The procedure in Example 16 was repeated, except that polyethyleneterephthalate (PET) film (Tetron V, 75 μm, manufactured by Teijin) wasused. The results are shown in Table 4.

                                      TABLE 4                                     __________________________________________________________________________           Stretched Film of Styrene Polymer                                             Production   Linear Expansion Coefficient                                                                    Surface                                        Process                                                                              Thickness                                                                           MD     TD         Roughness Ra.sup.1)                     No.    No.    (μm)                                                                             (°C..sup.-1)                                                                  (°C..sup.-1)                                                                 MD/TD                                                                              (μm)                                 __________________________________________________________________________    Example 16                                                                           Ref. Ex. 5(1)                                                                        80    3.2 × 10.sup.-5                                                                2.1 × 10.sup.-5                                                               1.5  0.013                                   Example 17                                                                           Ref. Ex. 5(1)                                                                        80    3.2 × 10.sup.-5                                                                2.1 × 10.sup.-5                                                               1.5  0.013                                   Example 18                                                                           Ref. Ex. 5(2)                                                                        80    2.4 × 10.sup.-5                                                                1.8 × 10.sup.-5                                                               1.3  0.015                                   Example 19                                                                           Ref. Ex. 5(3)                                                                        80    1.8 × 10.sup.-5                                                                1.5 × 10.sup.-5                                                               1.2  0.022                                   Com. Ex. 14                                                                          Ref. Ex. 5(4)                                                                        80    10.3 × 10.sup.-5                                                               5.6 × 10.sup.-5                                                               1.8  0.014                                   Com. Ex. 15                                                                          Ref. Ex. 5(5)                                                                        85    1.8 × 10.sup.-5                                                                6.8 × 10.sup.-5                                                               3.8  0.019                                   Com. Ex. 16                                                                          Ref. Ex. 5(6)                                                                        80    3.4 × 10.sup.-5                                                                2.8 × 10.sup.-5                                                               1.2  0.056                                   Com. Ex. 17                                                                          PET    75    2.5 × 10.sup.-5                                                                2.2 × 10.sup.-5                                                               1.1  0.02                                    __________________________________________________________________________           Magnetic Disk                                                                       Thickness of       Reliability                                          Magnetic                                                                            Magnetic Layer                                                                        Sliding Properties                                                                       Dimensional                                                                          Reproduction                           No.    Material                                                                            (μm) Running Properties.sup.2)                                                                Change.sup.3)                                                                        Conditions                             __________________________________________________________________________    Example 16                                                                           γ-Fe.sub.2 O.sub.3                                                            3       Good       Good   Good                                   Example 17                                                                           Co--Ni                                                                              0.3     Good       Good   Good                                   Example 18                                                                           γ-Fe.sub.2 O.sub.3                                                            3       Good       Good   Good                                   Example 19                                                                           γ-Fe.sub.2 O.sub.3                                                            3       Good       Good   Good                                   Com. Ex. 14                                                                          γ-Fe.sub.2 O.sub.3                                                            3       Good       Bad    Bad                                    Com. Ex. 15                                                                          γ-Fe.sub.2 O.sub.3                                                            3       Good       Bad    Bad                                    Com. Ex. 16                                                                          γ-Fe.sub.2 O.sub.3                                                            3       Good       Good   Bad                                    Com. Ex. 17                                                                          γ-Fe.sub.2 O.sub.3                                                            3       Good       Bad    Bad                                    __________________________________________________________________________     .sup.1) According to JIS B0601, cutoff: 0.08 mm                               .sup.2) Condition after repeated usage (× 100)                          Good.: LOAD can be readily conducted.                                         Bad: Needed to reset in disk drive system at least twice                      .sup.3) After 50 hours at 80° C., 70% RH                                Dimensional stability:                                                       Good: with < 0.1% deformation in any direction                                Bad: with ≧ 0.1* deformation in either direction                       Reproduction                                                                  Good: LOAD of program was good                                                Bad: LOAD of program was bad                                             

EXAMPLE 20

Based on 100 parts by weight of the styrene polymer obtained inReference Example 1 (1) which had been sufficiently dried under reducedpressure, 0.03 parts by weight of fluorescent whitening agent (OB-1,manufactured by Eastmann) and 12 parts by weight of fine-grain calciumcarbonate (average particle diameter, 0.9 μm; natural product, surfacetreated with stearic acid) were mixed using a super mixer for a minute,then extruded by a biaxial extruder with capillary die at the tipthereof and cut into pellet.

This pellet was stirred in a dry hot air at 120° C. This pellet wasextruded at 320° C. by a uniaxial extruder with a T-die at the tipthereof. The extruded molten sheet was taken up via cooling metalrollers of touch roll type. The temperature of the metallic rollers was70° C. and the average cooling rate was about 50 ° C./sec. The resulting1,000 μm thick pre-molded product was stretched between nip rolls withdifferent circumferential speeds by 3 times while heated at 115° C. Thestretching rate was 6,000%/min. Subsequently, this film was stretched by3 times at 120° C. and 3,000%/min. using a tenter, then heat treated at250° C. for 20 seconds under a state of tension.

Thickness of the film thus obtained was 120 μm. Witness measuredaccording to JIS L-1074 was 85. Linear expansion coefficient of the filmat 0° C. to 90° C. as measured by thermal mechanical analysis was 5×10⁻⁵° C., and heat shrinkage after 30 minutes at 200° C. was 0.5%. Elasticmodulus was 45,000 kg/cm².

One side of the base film thus obtained was corona treated and coatedwith magnetic coating composition. The constitution of this magneticcoating composition was as follows: γ-Fe₃ O₃ magnetic powder, 45 partsby weight; vinyl chloride--vinyl acetate copolymer (VAGH manufactured byU.C.C.), 17 parts by weight; acrylonitrile--butadiene copolymer (N 1432J manufactured by Nippon Zeon), 3.5 parts by weight; polyisocyanate(coronate L manufactured by Nippon polyurethane), 1.5 parts by weight,methyl isobutyl ketone, 50 parts by weight; toluene, 50 parts by weightand carbon black, 4 parts by weight. Thickness of the magnetic layerafter drying was 3 μm.

Picture was printed on the opposite side to the magnetic layer and inputand output of the information was attempted, providing good result. Thesurface did not glare, and the printed surface was presented vivid andpreferable appearance. The card had sufficient resiliency and itsappearance and performance were not affected by temperature change from0° to 100° C. The results are shown in Table 5.

EXAMPLE 21

The procedure in Example 20 was repeated, except that 42 parts by weightof titanium oxide (tipake A-100; an average particle diameter, 0.2 μm)was used as white inorganic filler and a magnetic card was obtained.Elastic modulus of this card was 52,000 kg/cm². The results re shown inTable 5.

COMPARATIVE EXAMPLE 18

The procedure in Example 20 was repeated, except tht white inorganicfiller was not used. The resulting magnetic card had same performance asthat of Example 20, but inferior in sliding properties. That is, afterrepeated usage, printed image was partially peeled off. The appearanceglared, and the card was sticky, providing bad touch. That is, theappearance was not preferable. The results are shown in Table 5.

COMPARATIVE EXAMPLE 19

The procedure in Example 20 was repeated by including 70% by weight ofwhite inorganic filler. As the result, the base film was not obtainedbecause it was broken during stretching.

COMPARATIVE EXAMPLE 20

A 100 μm thick vinyl chloride film containing 30% by weight of TiO₂ wasobtained. The procedure in Example 20 was repeated using this film. Theresulting magnetic card had insufficient heat resistance and dimensionalstability. The results are shown in Table 5.

COMPARATIVE EXAMPLE 21

The procedure in Example 20 was repeated, except that polyethyleneterephthalate having intrinsic viscosity of 0.61 (measured ino-chlorophenol solution at 35° C.) was used. The resulting magnetic cardhad low heat resistance and bad appearance, and particularly the surfaceglared, which was not preferred from the viewpoint of appearance.Elastic modulus of this card was 40,000 kg/cm². The results are shown inTable 5.

                                      TABLE 5                                     __________________________________________________________________________                            Linear.sup.2)                                                                           Dimensional                                              Inorganic                                                                           Content                                                                            Expansion                                                                           White-                                                                            Stability                                   No.    Resin.sup.1)                                                                        Filler                                                                              (wt %)                                                                             Coefficient                                                                         ness.sup.3)                                                                       to Heat                                                                              Appearance.sup.5)                                                                    Resilience.sup.6)             __________________________________________________________________________    Example 20                                                                           SPS   Calcium                                                                             11   5 × 10.sup.-5                                                                 84  Good   Good   Good                                       carbonate                                                        Example 21                                                                           SPS   Titanium                                                                            30   5 × 10.sup.-5                                                                 87  Good   Good   Good                                       oxide                                                            Com. Ex. 18                                                                          SPS   --    --   3 × 10.sup.-5                                                                 --  Good   Bad    Good                          Com. Ex. 20                                                                          Polyvinyl                                                                           Titanium                                                                            30   --    86  Bad    Good   Bad                                  chloride                                                                            oxide                                                            Com. Ex. 21                                                                          PET   Calcium                                                                             11   6 × 10.sup.-5                                                                 82  fair   Bad    Good                                       carbonate                                                        __________________________________________________________________________     .sup.1) SPS: Polystyrene having a syndiotactic configuration                  PET: Polyethylene terephthalate                                               .sup.2) measured at 0 to 90° C. by thermal mechanical analysis         .sup.3) measured according to JIS L1074                                       The sample of Comparative Example 1 was colorless and transparent             .sup.4) .sup.4 Heat shrinkage was measured at 200° C. for 30           minutes. within 0.5%: Good, 0.5-2%: Fair, ≧2%: Bad                     .sup.5) Good: The printed image had vivid color, and did not glare nor        adhere to hand                                                                Bad: Failed to satisfy any or all of the above requirements                   .sup.6) Compared by bending based on PET                                      The sample was cut into the same size as the commercially available           "telephone card", and bent in Ushape, the distance between the shorter        sides being 30 mm, then released, and distortion of the card was observed     Good: warp less than 2 mm                                                     Bad: warp over 2 mm                                                      

INDUSTRIAL AVAILABILITY

The above obtained magnetic recording medium of the present invention isexcellent in sliding properties and smoothness, has high heatresistance, moisture resistance and excellent dimentional stability totemperature and moisture, and further has good coersive force and can bedensified. In addition to the aforementioned characteristics, themagnetic tape of the present invention has good dimensional stability athigh temperature, and can provide good reproduction free from elongationof tape upon usage at high temperature of 90° C.

In addition to the above characteristics, the magnetic disk of thepresent invention has flexibility and less dimensional changes underhigh temperature and high humidity conditions, while dimentional changesin any direction may be well balanced. Moreover, sliding properties ofthe surface is extremely excellent.

The magnetic card of the present invention, which has, in addition tothe above characteristics, sufficient whiteness, is a resilient cardwith excellent appearance and durable upon usage for a long period oftime.

Accordingly, the magnetic recording medium of the present invention canbe suitably used widely, for example, as various kinds of magnetictapes, magnetic disks, magnetic drums, magnetic cards, etc. Further,according to the present invention, the magnetic tape can be efficientlyused as heat resistant magnetic tape which is used for on board audioapparatus such as car stereo or video; the magnetic disk as flexiblemagnetic disk such as floppy disk; and the magnetic card as variouskinds of magnetic cards such as prepaid card, ticket, card key and thelike.

We claim:
 1. A heat resistant magnetic tape which comprises a magneticlayer formed on at least one side of a base film which is a stretchedfilm consisting essentially of a styrene polymer having a syndiotacticconfiguration or a composition thereof or a laminate film containingsaid stretched film wherein elastic moduli in MD and TD at roomtemperature are not less than 30,000 kg/cm², the ratio (MD/TD) is notless than 0.8, elastic modulus at 90° C. in MD is not less than 10,000kg/cm², linear expansion coefficient at 0° to 90° C. is not more than 5×10⁻⁵ /° C. and surface roughness Ra is 0.001 to 0.03 μm.
 2. Themagnetic tape according to claim 1, which comprises a composition ofstyrene polymer having a syndiotactic configuration and containinginorganic filler.
 3. A magnetic disk which comprises a magnetic layerformed on at least one side of a base film which is a 20 to 200 μm thickstretched film consisting essentially of a styrene polymer having asyndiotactic configuration or a composition thereof or a laminate filmcontaining said stretched film wherein the linear expansion coefficientat 0° to 90° C. in the MD (α_(MD)) and that in the TD (α_(TD)) are notmore than 5×10⁻⁵ /° C., and the ratio (α_(MD) /α_(TD)) is 0.3 to 3 andsurface roughness Ra is 0.001 to 0.03 μm.
 4. The magnetic disk accordingto claim 3, wherein the stretched film comprises a composition of astyrene polymer having a syndiotactic configuration and containinginorganic filler.
 5. A magnetic card having a magnetic layer on at leastone side of a base film which is a film of a composition comprising 40to 99% by weight of a styrene polymer having a syndiotacticconfiguration and 60 to 1% by weight of white inorganic filler, and hasa linear expansion coefficient of not more than 7×10⁻⁵ /° C., awhiteness of not less than 75 and a thickness of 50 to 1,000 μm.